In many industries it is necessary to reduce pieces of material to fine powder. An example is coal which is reduced from nuggets to powder before being burned in certain types of power station furnace. Limestone, chalk and many other minerals must also, for most uses, be reduced to powder form.
Breaking up of the rock and grinding it into powder has, to the best of Applicant's knowledge, heretofore mainly been carried out mechanically. Ball mills, hammer mills and other mechanical structures which have moving parts that impact on, and hence crush, the pieces of material are widely used.
It has also been proposed that pieces of material should be broken up in a moving airstream. In prior U.S. Pat. No. 2,832,454 an airstream is blown at supersonic speed from a nozzle into a draft tube within which its speed falls to subsonic. Particles are sacked into the draft tube through an annular gap between the draft tube and the nozzle and broken up in the draft tube. In U.S. Pat. No. 5,765,766 pieces to be broken up fall into an airflow tube, are carried by the air flow into a disintegration chamber and blown against an anvil which breaks up the pieces. In both these structures the pieces are blown into the disintegration zone by air moving means upstream of the disintegration zone.
In U.S. Pat. No. 3,255,793 air is sucked by a centrifugal fan through a tube of circular and constant cross section. The tube is connected to the fan casing in which the fan rotor turns by a diverging conical nozzle. The United States specification states that the pieces entering the nozzle explode due to the fact that the air pressure in the nozzle is below the internal pressure of the particles.
The present invention seeks to provide a new pulveriser and a new method of pulverizing.